1. Field of the Invention
The invention relates to a hydraulic control apparatus for a vehicle and method thereof. More particularly, the invention relates to a hydraulic control apparatus including control means for controlling a rotational speed of an electric oil pump motor.
2. Description of the Related Art
A conventional hydraulic control apparatus for a vehicle includes an oil pump, and is configured to supply a hydraulic pressure generated by the oil pump to actuators of an automatic transmission and the like so as to drive the actuators, and to supply pressure oil to various portions of the vehicle so as to lubricate the portions.
Previously, an oil pump was configured so as to be directly connected to an input shaft extending from an engine to a transmission. In this configuration, an oil pump having a large discharge capacity was employed such that a required amount of oil for shift control can be obtained even a rotational speed of the input shaft was low. However, when the rotational speed of the input shaft increased, an excessive amount of oil was generated, which resulted in unnecessary consumption of a driving force generated by the pump.
Accordingly, there exists a conventional hydraulic control apparatus in which an oil pump is driven by an electric motor, and a rotational speed of the oil pump motor is controlled such that a required hydraulic pressure and a required flow volume of pressure oil can be obtained.
For example, a rotation control apparatus of an oil pump disclosed in Japanese Patent Laid-Open Publication No. 2000-18377 performs control for decreasing a rotational speed of the pump to a rotational speed corresponding to a minimum line pressure or less. This control is performed so as to prevent excessive consumption of energy, and noise caused by driving the pump, based on the fact that a required line pressure is lower than the minimum line pressure when next shifting is not performed immediately, for example, when a vehicle is stopped due to application of a barking force.
As another conventional hydraulic control apparatus, there exists a fluid pressure control apparatus for a hybrid vehicle including a belt type continuously variable automatic transmission, which is disclosed in Japanese Patent Laid-Open Publication No. 11-189073. The conventional apparatus includes an electric oil pump such that pressure oil can be supplied to the automatic transmission even when an engine is stopped. During steady running of the vehicle, the apparatus drives the electric motor of the oil pump at low speed such that the minimum amount of pressure oil for maintaining the present gear ratio can be obtained. During shifting of the automatic transmission, the apparatus performs control for driving the electric motor at high speed.
In the conventional hydraulic control apparatus, when an operation state of the vehicle (hereinafter, referred to as a “vehicle operation state”) is changed to a high load state, a rotational speed of the electric oil pump is returned from a low rotational speed state to a high rotational speed state in which a required line pressure can be generated, such that a required hydraulic pressure and a required amount of oil for shifting of the automatic transmission and engagement of a clutch mechanism are obtained. However, in a control state in which a vehicle speed and an engine speed are detected and the rotational speed of the electric oil pump is increased such that the required hydraulic pressure is generated, a problem may occur that the rotational speed of the electric pump is excessively low, which causes a temporary insufficiency of a hydraulic pressure in an early stage of a change in the vehicle operation state. For example, when actuators of the automatic transmission and the clutch mechanism are attempted to be driven before a rotational speed of an engine shaft is increased, the rotational speed of the oil pump is too low to increase the hydraulic pressure, and driving of the actuator is delayed. Accordingly, problems may occur, for example, shifting cannot be performed smoothly, and a belt slips in the case of the belt type continuously variable transmission.
In a hybrid vehicle in which two drive sources, that are an engine and a motor generator, are arranged, even when regenerative braking is being performed or power generation is being performed during running, a rotational speed of the oil pump motor needs to be increased from a low rotational speed region to a high rotational speed region such that an engagement state of a clutch of a power switching mechanism is changed, an appropriate gear ratio is obtained or a belt holding force of the CVT is increased. However, in the conventional hydraulic control apparatus, hydraulic control based on such a change in the operation state of the hybrid vehicle is not performed.
Also, in the conventional hydraulic control apparatus, a return speed is not adjusted at which the rotation of the oil pump is returned from the low rotational speed region to the high rotational speed region. Accordingly, a problem may occur that the rotational speed of the oil pump is further insufficient particularly in the early stage of the change in the operation state.